Many industrial assemblies involve a thin film of liquid being compressed in the interfacial region between two mating solid surfaces A typical case, in the context of which the present invention was developed, is the `premium connection` uniting two lengths of oilfield tubing in end-to-end relation. Such a connection comprises two externally threaded pipe ends (pins) screwed tightly into each end of an internally threaded coupling (box). The pins carry annular lands which press against annular lands carried by the boxes to effect circumferential pressure seals remote from the threads. A lubricant/sealant, referred to as thread compound or "dope", is disposed as a thin film between the mating parts. The term `liquid film` is intended herein to encompass a pure liquid, or a grease, or a slurry of solids in liquid or grease (of which thread compound is an example).
The liquid film is subjected to a normal compressive stress or "contact pressure". Contact pressure can be defined as the compressive stress between two solid bodies, normal to the interface surfaces thereof, which is created by the application of external force to said bodies.
Determining the contact pressure between mating parts is of importance for general stress analysis and in particular for ascertaining the effectiveness of the seal being created. However, present methods for doing this are only of limited usefulness. The methods available can be characterized as `direct` or `indirect`.
Indirect methods involve first developing a stress analysis model by photoelastic, analytical or numeric means. A load is then applied to the specimen and external measurements of stress, strain or applied load are made using instruments such as a strain gauge. The stress analysis model and external measurements are then combined to estimate the internal contact pressure of the specimen. These methods can at best give a measure of the average contact pressure and cannot account for variations arising from actual geometrics or other variables versus modelled assumptions. Furthermore few three dimensional effects can be inferred.
There is only one existing direct method available for this purpose, to the applicant's knowledge. This involves neutron diffraction strain measurement The method is directed toward measuring the strain between crystal lattice planes inside the solid parts. If the region examined is directly inside a loaded contact surface, the contact pressure can be inferred from the relationship between strain and stress However this procedure is time consuming to carry out and involves very complex instruments and calculations that are not suited for use outside a specialized laboratory.
So there presently exists a need for a means that can be used in the field to directly measure the internal contact pressure on the surfaces of mating solid parts. In a narrower sense, there exists a need in the oilfield tubing premium connection art for a means that can be used when making up the threaded parts to measure the metal-to-metal sealing surface contact pressures and determine when they reach a desired pre-stress.
The present invention involves the adaption of existing ultrasonic flaw detection technology in a unique way to the problem of directly measuring internal contact pressure.